1. Technical Field
The present invention relates to a white ink composition and a recorded matter obtained using the same.
2. Related Art
In image recording such as ink jet printing, usually, as a white coloring material, for example, a metal oxide, such as titanium dioxide, zinc oxide, silica, alumina, or magnesium oxide, or a white pigment, such as barium sulfate or calcium carbonate, is used. A white ink composition containing hollow resin particles as a white coloring material is also known (for example, see U.S. Pat. No. 4,880,465). In such a constitution, the internal cavities of the hollow resin particles are filled with a solvent in the ink composition. As a result, since the specific gravity of the hollow resin particles becomes substantially the same as that of the ink composition, the hollow resin particles can be stably dispersed in the ink composition. When an image formed on a recording medium using this ink composition, since the internal space of the hollow resin particles is replaced with air during drying, the hollow resin particles can exhibit a light-shielding effect due to light scattering caused by a difference in refractive index between the envelope and the cavity of the particle.
The ink composition contains a fixing resin for fixing the coloring material. JP-A-2009-138077 discloses a white ink composition containing hollow resin particles, serving as a white material, and a polyurethane resin having a glass transition temperature of 50° C. or less, serving as a fixing resin, and describes that such a constitution enables to form white images excellent in abrasion resistance. In addition, JP-A-2006-96933 discloses an ink jet recording ink containing a pigment and a fixing resin component, for fixing the pigment, composed of two types of polymers: an amphiphilic polymer and a polymer having a glass transition temperature (Tg) of −30 to 60° C. JP-A-2006-96933 describes that, in such a constitution, the ink jet recording ink can form letters and images excellent in water resistance, abrasion resistance, and bleeding resistance on various recording media and has satisfactory discharge stability. Thus, the performance of an ink composition and the quality of images formed thereby highly depend on the kind and blending conditions of the fixing resin component contained in the ink composition.
In some cases, the white ink composition is used to eliminate the background color for enhancing color development of a color image when the color image is formed on a medium having a background color that is not necessarily white, such as plastic products and metal products. Alternatively, in a case of forming a color image on a transparent sheet, the white ink composition may be used to form a white shielding layer for decreasing the light transmission of the color image. However, formation of an image by process color inks after formation of a base layer by the white ink composition has a problem that color inks applied onto the white image tend to bleed.
The present inventors have found that a urethane resin can impart excellent abrasion resistance to images, but make the surfaces of the images sticky, which may cause adhesion between the images when the recorded matter are stacked.